We investigate the anisotropic frequency-dependent dielectric, THz and IR response of liquid water confined between two planar graphene sheets with force-field- and density-functional-theory-based molecular dynamics simulations. Using spatially resolved anisotropic spectra, we demonstrate the critical role of the volume over which the spectral response is integrated when reporting spatially averaged electric susceptibilities. To analyze the spectra, we introduce a unique decomposition into bulk, interfacial, and confinement contributions, which reveals that confinement effects on the spectra occur only for systems with graphene separation below 1.4 nm, for all frequencies. Based on this decomposition, we discuss the molecular origin of the main absorption features of nanoconfined water from the GHz to the IR regime. We show that, at low frequencies, the 15 GHz Debye peak of interfacial water is redshifted due to a slowdown of collective water reorientations. At high frequencies, the OH stretch at 100 THz blue shifts and a signature of free OH groups emerges, while the HOH bend mode at 50 THz is redshifted. Strikingly, in nanoconfinement, the 20 THz libration band shifts to below 15 THz and broadens drastically, spanning two orders of magnitude in frequency. These results are rationalized by the collective water motion and the structure of the hydrogen-bond network at the water–graphene interface and in two-dimensional water layers, which reveals the intricate behavior of nanoconfined water and its spectral properties.
